#[BlockSupport]
Attribute on classes.
Declares a custom block support: blocks of the same theme or plugin that
use it with #[UseSupport] get an attribute, a control in the inspector and
the class names it returns on their root element.
The class is a service with a public method className(). Its only
parameter is the value of the attribute: its type (int, float, string, bool
or a backed enum, optionally nullable) derives the schema and the control,
its default is the default of the attribute. The method returns the class
names for the value (separated by spaces), or null or '' for none; it runs
while the block renders, so the service may depend on other services.